For determining a mass flow rate of a fluid, especially a liquid, flowing in a pipeline, it is common to use measuring devices, which bring-about Coriolis forces in the fluid by means of a measuring transducer of vibration-type and a control and evaluation electronics connected thereto and which produce, derived from these forces, a measurement signal representing the mass flow rate.
Such Coriolis mass flow meters have been known for a long time and are established in industrial applications. Thus, e.g. in DE-A 39 16 285, EP-A 317 340, EP-A 518 124, EP-A 1 207 375, U.S. Pat. Nos. 4,823,614, 5,291,792, 5,398,554, 5,476,013, 5,531,126, 5,691,485, 5,705,754, 5,796,012, 5,945,609, 5,979,246, 6,006,609, 6,223,605, 6,484,591, WO-A 99 51 946, WO-A 99 40 394 OR WO-A 00 14 485, Coriolis mass flow meters are described, each with a measuring transducer of vibration-type. The measuring transducer reacts to a mass flow rate of a fluid flowing in a pipeline and includes:                a single, curved, or straight, measuring tube to convey the fluid and vibrate in operation, which measuring tube communicates with the pipeline via an inlet tube piece opening on the measuring tube inlet end and an outlet tube piece opening on the measuring tube outlet end,        a support element fixed to the inlet and outlet ends of the measuring tube, as well as        an exciter arrangement, which excites the measuring tube during operation mainly to bending oscillations in a tube plane, as well as        a sensor arrangement for the point registering of inlet and outlet end oscillations of the measuring tube.        
Vibrating measuring tubes of such measurement pickups are known to cause, when excited to lateral bending oscillations about an imaginary oscillation axis connecting the inlet and outlet ends, Coriolis forces in the through-flowing fluid. These, in turn, lead to a superimposing of coplanar bending oscillations of a second eigenoscillation form of higher and/or lower order, the so-called Coriolis mode, onto the excited bending oscillations in the so-called wanted mode, and, as a result, the oscillations registered by means of the sensor arrangement at the inlet and outlet ends exhibit a measurable phase difference dependent on, among other things, the mass flow rate.
Usually, the measuring tubes of such measuring transducers, e.g. as utilized in Coriolis mass flow meters, are excited, during operation, to an instantaneous resonance frequency of a natural eigenoscillation form, especially in the case of oscillation amplitude which is controlled to be constant. Since this resonance frequency is dependent especially also on the instantaneous density of the fluid, usual Coriolis mass flow meters on the market can also be used to measure, besides the mass flow rate, also the density of flowing fluids.
A significant disadvantage of the above-described measuring transducers lies in the fact that, due to alternating, lateral deflections of the single measuring tube vibrating predominantly in the wanted mode, oscillatory transverse forces of the same frequency can act on the pipeline and these transverse forces can only be compensated, independently of the density of the medium to be measured, by very great technical effort.
For improving the dynamic balance of the measuring transducer, especially for reducing such transverse forces produced by the vibrating, single, measuring tube and acting on the pipeline at the inlet and outlet ends of the measuring tube, the measuring transducers shown in EP-A 317 340, U.S. Pat. Nos. 5,398,554, 5,531,126, 5,691,485, 5,796,012, 5,979,246 or WO-A 00 14 485 each include at least one one-piece, or multi-part, counteroscillator, which, in each case, is fixed to the measuring tube at the inlet and outlet ends. Such beam-shaped, especially tubular, counteroscillators, or those counteroscillators realized as body pendula aligned with the measuring tube, oscillate, during operation, out of phase, especially with opposite phase, to that of the associated measuring tube, whereby the action of the transverse forces evoked, respectively, by measuring tube and counteroscillator on the pipeline can be minimized and possibly even completely suppressed.
Such measuring transducers with counteroscillator have proven themselves especially for applications where the fluid to be measured has an essentially constant, or only very slightly changing, density, thus for applications where a resulting force acting on the connected pipeline and derived from the transverse forces produced by the measuring tube and the opposing forces produced by the counteroscillator can initially be adjusted, without more, fixedly to zero.
In contrast, a measuring transducer of such type, especially according to U.S. Pat. No. 5,969,265, when applied for fluids with density fluctuating over a wide range, e.g. different fluids to be measured, sequentially following one another, has, while perhaps in lesser degree, practically the same disadvantage as a measuring transducer without counteroscillator, since the resultants of the transverse forces are also dependent on the density of the fluid and, therefore, can vary in significant measure from zero. Said differently, even a total system composed of measuring tube and counteroscillator is, during operation, globally deflected from an assigned, static, rest position on the basis of density dependent imbalances and transverse forces associated therewith.
Further possibilities also for the density independent reduction of such transverse forces are proposed e.g. in U.S. Pat. No. 5,531,126, WO-A 99 40 394 or WO-A 00 14 485.
In particular, a measuring transducer of vibration-type for a fluid flowing in a pipeline is described in WO-A 00 14 485 and includes:                a measuring tube for conveying the fluid, inserted into the course of the pipeline and vibrating at least at times during operation;        a first, especially likewise vibrating, support element affixed to an inlet end and to an outlet end of the measuring tube;        an electromechanical exciter arrangement for producing an exciter force variable as a function of time, for causing the measuring tube to vibrate;        a sensor arrangement for registering oscillations of the measuring tube; as well as        a coupler arrangement connected with measuring tube and support element and having two coupling elements electromechanically interacting with the vibrating measuring tube and the support element for producing balancing oscillations;        wherein the measuring tube, in operation, is excited by means of the exciter arrangement at least at times to an oscillation mode,        in which it executes such bending oscillations about an oscillation axis imaginarily connecting the inlet end and the outlet end of the measuring tube, that it assumes mainly an oscillation form having a single bending-oscillation antinode, and        in which it is laterally displaced at least at times out of an assigned, static, resting position due to concurrently produced transverse forces; and        wherein the balancing oscillations of the coupling elements are so developed and arranged on the measuring tube, that the produced transverse forces are compensated and, consequently, a center of mass of an oscillation system formed of measuring tube, exciter arrangement, sensor arrangement and the two cantilevers is maintained in one position.        
Furthermore, a measuring transducer of vibration-type for a fluid flowing in a pipeline is described in WO-A 99 40 394.
This transducer includes:                a measuring tube for conveying the fluid, inserted into the course of the pipeline and vibrating at least at times during operation,        wherein the measuring tube communicates with the pipeline via a first connecting tube piece opening into the inlet end and via a second connecting tube piece opening into the outlet end and aligned with the first connecting tube piece and with the oscillation axis;        a support element fixed on an inlet end and on an outlet end of the measuring tube and adapted to serve as a likewise vibrating counteroscillator;        a second support element adapted to serve as a transducer housing;        an electromechanical exciter arrangement for producing an exciting force variable with time for causing the measuring tube to vibrate;        a sensor arrangement for registering oscillations of the measuring tube; as well as        a coupler arrangement connected with the measuring tube, the two support elements, as well as the connecting tube pieces and having two coupling elements fixed to the vibrating measuring tube, the two support elements, and the connecting tube pieces, for transferring balancing forces into the transducer housing;        wherein the measuring tube is excited in operation by means of the exciter arrangement, at least at times, to an oscillation mode,        in which it executes bending oscillations about an imaginary oscillation axis connecting the inlet end and the outlet end of the measuring tube, such that it assumes mainly a form of oscillation having a single bending oscillation antinode;        wherein the two coupling elements of the coupler arrangement are mechanically coupled with the measuring tube and the support element such that each of the two coupling elements act in a region of a common oscillation node of measuring tube and counteroscillator; and        wherein a constant spring constant of the coupler arrangement determined by the coupling elements and acting between measuring tube and transducer housing is chosen sufficiently high that the coupler arrangement acts in an effective direction of the transverse forces essentially as a rigid body.        
Moreover, a measuring transducer of vibration-type for a fluid flowing in a pipeline is shown in U.S. Pat. No. 5,531,126 to include:                a measuring tube for conveying the fluid, inserted into the course of the pipeline and vibrating at least at times during operation;        a first, especially likewise vibrating, support element affixed to an inlet end and to an outlet end of the measuring tube;        an electromechanical exciter arrangement for producing an exciter force, variable as a function of time, for causing the measuring tube to vibrate;        a sensor arrangement for registering oscillations of the measuring tube; as well as        a coupler arrangement connected with measuring tube and support element and having a coupling element electromechanically interacting with the vibrating measuring tube and the support element;        wherein the measuring tube, in operation, is excited by means of the exciter arrangement at least at times to an oscillation mode,        in which it executes such bending oscillations about an oscillation axis imaginarily connecting the inlet end and the outlet end of the measuring tube, that it assumes mainly an oscillation form having a single bending-oscillation antinode;        wherein the one coupling element is mechanically coupled with the measuring tube and the support element such that it acts on the measuring tube in a central region of the single oscillation antinode; and        wherein an adjustable spring constant of the coupler arrangement determined by the coupling element and acting between measuring tube and support element is made to be negative.        
In the case of the aforementioned measuring transducers, the problem of density-dependent imbalances is solved, in principle, by matching an amplitude response of the coupling elements or the counteroscillator, by means of variable spring constants dependent on the instantaneous oscillation amplitude, to the measuring tube oscillations during operation, in such a way that the transverse forces produced by the vibrating measuring tube and counteroscillator partially cancel one another. However, a disadvantage of the proposed measuring transducer is, in such cases, that despite technically always very complicated designs of the compensation mechanism, a complete neutralizing of the transverse forces produced by the measuring tube is neither practically nor theoretically possible.
Another possibility for reducing density-dependent, transverse forces is described in e.g. U.S. Pat. Nos. 5,287,754, 5,705,754 or U.S. Pat. No. 5,796,010. In the case of the measuring transducers shown in these patents, the transverse forces oscillating more at medium or high frequency, produced by the single, vibrating measuring tube, are kept away from the pipeline by means of a counteroscillator, which, compared to the measuring tube, is very heavy, and by means of, if necessary, a relatively soft coupling of the measuring tube to the pipeline, thus, in effect, by means of a mechanical, low-pass filter. A great disadvantage of such a measuring transducer lies, among other things, however, in the fact that the mass of the counteroscillator required for achieving a sufficiently robust damping increases more than proportionally with the nominal diameter of the measuring tube. An application of such massive structural components means, on the one hand, always an increased complexity of assembly, both in the manufacture and in the installing of the measuring device into the pipeline. On the other hand, in such case, it must always be assured that a minimum eigenfrequency of the measuring transducer, which becomes increasingly lower with increasing mass, lies, as before, far from the likewise very low eigenfrequencies of the connected pipeline. Consequently, a use of such a measuring transducer in industrially installable Coriolis mass flow meters, especially for measurements of liquids, or also in Coriolis mass flow/density meters, is most likely limited to relatively small nominal diameters of less than, or equal to, 10 mm.
Moreover, other disturbances of the described kind can, furthermore, arise, as, for example, also discussed in U.S. Pat. No. 5,291,792, in that, in addition to the oscillations in the wanted and Coriolis modes, also lateral, bending-oscillation modes can be excited, whose oscillation directions lie outside of, for example perpendicular to, the shared oscillation direction of the wanted and Coriolis modes.